1. Field of the Invention
This invention relates to thermal-insulating coatings for components exposed to high temperatures, such as the hostile thermal environment of a gas turbine engine. More particularly, this invention is directed to a thermal barrier coating (TBC) containing elemental carbon and/or carbides in yttria-stabilized zirconia that has been alloyed with at least a third oxide to increase the lattice strain energy of the TBC grains. The resulting TBC is characterized by lower density and thermal conductivity, high temperature stability, and improved mechanical properties from a hardening effect between carbon and the third oxide.
2. Description of the Related Art
Higher operating temperatures for gas turbine engines are continuously sought in order to increase their efficiency. However, as operating temperatures increase, the high temperature durability of the components within the hot gas path of the engine must correspondingly increase. Significant advances in high temperature capabilities have been achieved through the formulation of nickel and cobalt-base superalloys. Nonetheless, when used to form components of the turbine, combustor and augmentor sections of a gas turbine engine, such alloys alone are often susceptible to damage by oxidation and hot corrosion attack, and as a result may not retain adequate mechanical properties. For this reason, these components are often protected by a thermal barrier coating (TBC) system. TBC systems typically include an environmentally-protective bond coat and a thermal-insulating topcoat, typically referred to as the TBC. Bond coat materials widely used in TBC systems include oxidation-resistant overlay coatings such as MCrAlX (where M is iron, cobalt and/or nickel, and X is yttrium or another rare earth or reactive element), and oxidation-resistant diffusion coatings such as diffusion aluminides that contain nickel-aluminum (NiAl) intermetallics.
TBC materials are typically ceramic materials and particularly zirconia (ZrO2) that is partially or fully stabilized by yttria (Y2O3), magnesia (MgO), ceria (CeO2), calcia (CaO), scandia (Sc2O3) or other oxides. Binary yttria-stabilized zirconia (YSZ) is widely used as a TBC material because of its high temperature capability, low thermal conductivity and erosion resistance in comparison to zirconia stabilized by other oxides, e.g., ceria-stabilized zirconia, which exhibits poorer erosion resistance as a result of being relatively soft. YSZ is also preferred as a result of the relative ease with which it can be deposited by plasma spraying, flame spraying and physical vapor deposition (PVD) techniques. In plasma spraying processes, the coating material is typically in the form of a powder that is melted by a plasma as it leaves a spray gun. As a result, a plasma-sprayed TBC is formed by a buildup of molten xe2x80x9csplatsxe2x80x9d and has a microstructure characterized by irregular flattened grains and a degree of inhomogeneity and porosity. TBC""s employed in the highest temperature regions of gas turbine engines are often deposited by electron beam physical vapor deposition (EBPVD), which yields a columnar, strain-tolerant grain structure that is able to expand and contract without causing damaging stresses that lead to spallation. Similar columnar microstructures can be produced using other atomic and molecular vapor processes, such as sputtering (e.g., high and low pressure, standard or collimated plume), ion plasma deposition, and all forms of melting and evaporation deposition processes (e.g., cathodic arc, laser melting, etc.).
In order for a TBC to remain effective throughout the planned life cycle of the component it protects, it is important that the TBC has and maintains a low thermal conductivity throughout the life of the component, including during high temperature excursions. However, the thermal conductivities of TBC materials such as YSZ are known to increase over time when subjected to the operating environment of a gas turbine engine. As a result, TBC""s for gas turbine engine components are often deposited to a greater thickness than would otherwise be necessary. Alternatively, internally cooled components such as blades and nozzles must be designed to have higher cooling flow. Both of these solutions are undesirable for reasons relating to cost, component life and engine efficiency. As a result, it can be appreciated that further improvements in TBC technology are desirable, particularly as TBC""s are employed to thermally insulate components intended for more demanding engine designs.
U.S. Pat. No. 5,906,895 to Hamada et al. discloses a method of inhibiting the deterioration of the thermal properties of a TBC by suppressing a reaction sintering mechanism said to occur in TBC""s at high temperatures. In Hamada et al., a high temperature compound (such as a carbide, nitride or another high temperature material) is said to be compounded into a YSZ TBC deposited by a plasma spraying process. According to three plasma spraying techniques disclosed by Hamada et al., the high temperature compound appears to be present as splats dispersed within the TBC as a result of the plasma spraying process. According to a fourth technique disclosed by Hamada et al., a plasma-sprayed TBC is infiltrated with a feed gas of the high temperature compound, apparently forming a coating of the compound on the inter-splat boundaries of the porous TBC. Following this treatment, any remaining feed gas would inherently escape the TBC through the same passages that allowed the gas to infiltrate the TBC. With each approach, the high temperature compound is said to suppress reaction sintering of the YSZ TBC by some unexplained mechanism.
In commonly-assigned U.S. Pat. No. 6,492,038 to Rigney et al., a more thermally-stable TBC is achieved by inhibiting grain growth (coarsening), sintering, and pore redistribution (the coalescence or coarsening of smaller pores to form larger pores) during high temperature excursions. According to Rigney et al., resistance to heat transfer through a TBC is determined in part by the amount of microstructural defects within the grains of the TBC. Rigney et al. teach that such defects can be created by composition-induced defect reactions and process-induced porosity, the former of which includes vacancies that result from the need in ionic solids to maintain charge neutrality, as is the case in YSZ where substitution of zirconia (ZrO2) with yttria (Y2O3) in the lattice yields a vacancy. On the other hand, process-induced porosity includes pore formation that occurs during coating as a component is rotated relative to the deposition source. A primary example is the xe2x80x9csunrise-sunsetxe2x80x9d vapor-surface mechanisms that occur during rotation of a component during deposition of TBC from a vapor cloud, such as by PVD, the result of which is a textured growth of the deposit in which pores are formed between columns, within the columns, and between secondary growth arms contained within the columns.
Rigney et al. teach a technique by which process-induced porosity in a TBC is preserved by incorporating extremely fine precipitates into the TBC microstructure. More particularly, Rigney et al. teach that limited amounts of extremely fine carbide and/or nitride precipitates formed at the defects, pores and grain boundaries of the TBC microstructure serve to pin the TBC grain boundaries to inhibit sintering, grain coarsening, and pore redistribution during high temperature excursions, with the effect that the microstructure, and consequently the thermal conductivity of the TBC, is stabilized. Rigney et al. teach that suitable carbiding/nitriding techniques include depositing the TBC using a physical vapor deposition technique in an atmosphere that contains carbon and/or nitrogen vapors, gases or compounds, and/or heat-treating in the presence of a gas containing carbon and/or nitrogen gases or compounds. Contrary to Hamada et al., the carbide/nitride precipitates must be incorporated as extremely fine precipitates in order to pin the TBC grain boundaries.
While the incorporation of carbide/nitride precipitates in accordance with Rigney et al. makes possible a more stabilized TBC microstructures, further improvements in TBC microstructure and processes would be desirable.
The present invention generally provides a thermal barrier coating (TBC) and method for forming the coating on a component intended for use in a hostile environment, such as the superalloy turbine, combustor and augmentor components of a gas turbine engine. The method of this invention is particularly directed to producing a TBC characterized by lower density and thermal conductivity, high temperature stability, and improved mechanical properties. Improvements obtained by this invention are particularly evident with TBC having a columnar grain structure, such as those deposited by EBPVD and other PVD techniques, though the invention is also applicable to TBC deposited by such methods as plasma spraying.
The invention generally entails a TBC formed of a thermal-insulating material that contains yttria-stabilized zirconia (YSZ) alloyed with at least a third oxide. The TBC is formed to also contain elemental carbon, and may potentially contain carbides and/or a carbon-containing gas that forms from the thermal decomposition of carbon. According to the invention, the TBC is characterized by lower density and thermal conductivity, high temperature stability and improved mechanical properties. To exhibit the desired effect, the third oxide is more particularly one that increases the lattice strain energy of the TBC microstructure as a result of having an ion size that is sufficiently different from a zirconium ion (Zr4+). While not wishing to be held to any particular theory, increased strain energies are believed to act as scattering sites for lattice vibrations (phonons) that contribute to the thermal conductivity of YSZ. It is believed that metal oxides having an absolute percent ion size difference relative to zirconium ions of at least that of the yttrium anion (Y3+), i.e., at least 13 percent, are effective to produce significant strains due to ionic size. Oxides of metals such as cerium, gadolinium, neodymium, lanthanum, dysprosium, tantalum, magnesium, calcium, strontium and barium meet this requirement, a particular example being ceria (CeO2) which has an absolute percent ion size difference relative to zirconium ions of about 30 percent. However, ceria additions to YSZ are known to have the undesirable effect of increasing coating density and reducing erosion resistance. In the present invention, it has been unexpectedly shown that a ceria-modified YSZ coating that is further modified by doping with carbon undergoes hardening during thermal aging as an apparent result of a reaction that occurs between ceria and the elemental carbon present in the TBC. Advantageously, density of the coating is also decreased due to the carbon content forming small and stable pores when the coating is sufficiently heated.
Other objects and advantages of this invention will be better appreciated from the following detailed description.